(a) Field of the Invention
This invention relates to a column filter which uses bundles of long fibers as a filter medium to remove at a high rate suspended solids from raw liquids such as city water, industrial water, sewage, river water, lake or pond water, supernatant waters from coagulation and settling treatments, waters discharged intermediately during the practice of various processes, recovered waters such as those from pulp-and paper-making processes, various waste waters, processing waters or valuable-material-containing liquids from biological treatment apparatus, alcoholic beverages, oils and the like. In particular, this invention is concerned with a column filter of the above sort, in which an improvement has been applied to the structure holding lower end portions of the bundles of long fibers.
(b) Description of the Related Art
A variety of filters have conventionally been used to remove suspended solids from a raw liquid.
They may be classified into several different types, including filters adapted for the removal of relatively large suspended solids from a raw liquid by a screen such as a wire mesh, filters employed for the microfiltration of fine particles from a raw liquid by a filter membrane having micropores, and filters using filter elements, which carry a precoat layer of a filter aid coated thereon, for the removal of suspended solids.
Various filters are thus used depending on the kinds of raw liquids and the purposes of their filtration. Among these, filters of the packed medium type are most common.
A typical packed medium filter has a cylindrical upright shell with a packed layer of a granular filter medium such as filter sand or anthracite or a fibrous filter medium such as short fibers or fiber balls. A raw liquid is passed as a downflow or upflow through the packed medium layer so that solids suspended in the raw liquid may be trapped in the packed layer to obtain a filtered liquid, namely, filtrate. Filtration is stopped when the pressure drop has increased to a predetermined level or the quality of the filtered liquid has fallen to a prescribed level. The packed medium layer is then flushed with water or expanded or agitated by a water stream, and air or the like, so that the solids thus trapped may be flushed away to permit resumption of the filtration through the packed medium layer thus flushed.
With packed medium filters it is desired that in the filtration step the pressure drop due to the filter medium resistance does not increase substantially and that the quality of the filtered liquid remains good even when filtration is at a high flow rate; and that in the washing step the solids trapped in the packed medium layer can be flushed completely with a minimum of both water and time.
A filter using a conventional granular filter medium such as sand or anthracite is however accompanied by drawbacks--feed velocity of the liquid through the filter medium can be as low as 20 m/hr even at the maximum, and, depending on the kind of the suspended solids, the filter medium is prone to prematured clogging and results in an increased pressure drop at a relatively early stage. During washing, this filter develops further drawbacks--washing must be at a flow rate slow enough to prevent the granular filter medium from flowing out of the shell, and the washing thus tends to be insufficient to prevent the gradual accumulation of suspended solids and, in some instances, the formation of mud balls or the like may result. There are still further drawbacks in that a relatively large quantity of washing water is required and the level of concentration of suspended solids in the effluent of washing is small.
A filter using a medium such as short fibers or fibrous balls allows free choice in size of the filter medium. It thus has an advantage that an optimal filter medium can be chosen depending on the kind of solids suspended in a raw liquid. Because the filter medium used has an extremely light weight, the filter is accompanied by drawbacks; for example, a screen or the like must be provided to prevent the filter medium from flowing out of the shell during the washing, the structure of the filter tends to become complex, and the screen becomes an obstacle to hamper the removal of trapped solids from the shell during washing, so that the washing cannot be effected sufficiently, resulting in the gradual accumulation of trapped solids.
A high rate filter--which solves the above-described drawbacks of conventional packed medium filters, permits filtration at a high rate without any substantial increase in the pressure drop by filtration resistance, and can be washed completely with a small quantity of washing water in a short time, by using a filter with bundles of long fibers--has already been proposed by the present inventors in Japanese Patent Application No. 146989/1987.
The above-proposed high rate filter has a support provided transversely or packed within a shell and bundles of long fibers 400-1,000 mm long fixed at lower end portions thereof on an upper part of the support and free-standing at upper end portions thereof, whereby a filter medium is formed. A raw liquid is passed as a downflow from the upper end portions of the fibers toward the lower end portions thereof, so that suspended solids may be trapped in interstitial spaces among the long fibers.
The use of a bundled long fiber filter has various advantages unavailable from the use of filters packed with conventional filter sand or anthracite; for example, a high filtration rate, pressure drop does not increase to any substantial extent, and complete washing is possible with a smaller quantity of wash water and in a shorter time. However, the filter has the problem that the operable filtration time is rather short, which is not satisfactory.